Heterojunction field-effect transistors (FETs), also known as high electron mobility transistors (HEMTs), are field-effect transistors that utilize a junction between two materials with different bandgaps (i.e., a heterojunction) to provide a channel for conduction. Compared to conventional FETs, heterojunction FETs are generally able to operate at higher frequencies with better performance and thus are often used in wireless communications devices such as base stations and mobile phones.
FIG. 1 illustrates a cross-sectional view of a conventional heterojunction FET 10. The conventional heterojunction FET 10 includes a substrate 12, a barrier layer 14 over the substrate 12, and a cap layer 16 over the barrier layer 14. A source region 18 and a drain region 20 are in the cap layer 16, the barrier layer 14, and the substrate 12 and laterally separated from one another such that a portion of the cap layer 16, the barrier layer 14, and the substrate 12 are located between the source region 18 and the drain region 20. A gate contact 22 is on the cap layer 16 opposite the barrier layer 14. A source contact 24 is on the source region 18, and a drain contact 26 is on the drain region 20.
The bandgap and asymmetric bonding energy of the material of the substrate 12 is different than the bandgap and asymmetric bonding energy of the material of the barrier layer 14. This polar asymmetry and bandgap difference at the junction between the substrate 12 and the barrier layer 14 induces a two-dimensional electron gas in the substrate 12 at this interface. The charge at this interface is caused by two mechanisms: spontaneous polarization and piezoelectric polarization. Spontaneous polarization occurs due to an intrinsic asymmetry of the bonding in the equilibrium wurtzite crystal structure, while piezoelectric polarization is caused by mechanical stress in the different materials generated by lattice mismatches between the different materials of the substrate 12 and the barrier layer 14. This two-dimensional electron gas provides a channel for the conventional heterojunction FET 10. Depending on a thickness of the barrier layer 14 and other design factors, the two-dimensional gas may be provided such that the conventional heterojunction FET 10 is normally on (i.e., the two-dimensional gas is sufficient to provide conduction between the drain contact 24 and the source contact 26 at steady state) or normally off (i.e., the two-dimensional gas is not sufficient to provide conduction between the drain contact 26 and the source contact 24 at steady state).
If the conventional heterojunction FET 10 is normally off, a voltage applied to the gate contact 22 is used to cause the accumulation of additional electrons at the heterojunction between the substrate 12 and the barrier layer 14, effectively densifying the two-dimensional gas and increasing the conductivity of the channel. When the voltage applied to the gate contact 22 is above a threshold voltage, the two-dimensional gas is sufficient to cause conduction between the drain contact 26 and the source contact 24, thereby turning the device on. If the conventional heterojunction FET 10 is normally on, a negative voltage applied to the gate contact 22 may be used to cause depletion of the electrons at the heterojunction between the substrate 12 and the barrier layer 14, effectively reducing the density of the two-dimensional electron gas and reducing the conductivity of the channel. When the negative voltage applied to the gate contact 22 is more negative than a threshold voltage, the two-dimensional gas is no longer sufficient to cause conduction between the drain contact 26 and the source contact 24, thereby turning the device off.
The substrate 12 is intrinsic (i.e., undoped) gallium nitride. The barrier layer 14 is aluminum gallium nitride. The cap layer 16 is gallium nitride, but is generally not intrinsic. Generally, the ratio of aluminum to gallium in the barrier layer 14 determines a sheet charge (Ns) of the two-dimensional gas at the heterojunction between the substrate 12 and the barrier layer 14, where the sheet charge contributes to the on-state resistance and thus output power achievable by the device. Due to increasing output power requirements in many applications, it may be desirable to maximize the sheet charge of the two-dimensional electron gas.
One way to increase the sheet charge of the two-dimensional electron gas is by increasing the ratio of aluminum to gallium in the barrier layer 14. As an example, while a barrier layer 14 comprising Al0.23Ga0.77N may provide a sheet charge of 9×1012 cm−2, a barrier layer 14 comprising pure aluminum nitride may provide a significantly larger sheet charge of 4.6×1013 cm−2. However, as the aluminum content in the barrier layer 14 increases, the reliability of the conventional heterojunction FET 10 decreases. This is because adding additional aluminum to the barrier layer 14 generates more strain in the conventional heterojunction FET 10 as a result of further lattice mismatching between the substrate 12 and the barrier layer 14. This in turn accelerates deterioration and failure of the conventional heterojunction FET 10. For example, a barrier layer 14 comprising Al0.23Ga0.77N may result in the conventional heterojunction FET 10 having a lifetime of more than a million hours, while a barrier layer 14 comprising Al0.5Ga0.5N may result in the conventional heterojunction FET 10 having a lifetime that is several orders of magnitude less.
One way to reduce strain in the conventional heterojunction FET 10 is by providing a relaxation layer 28 over the substrate 12 and a strained channel layer 30 between the relaxation layer 28 and the barrier layer 14 as shown in FIG. 2 to provide a conventional double heterojunction FET 32. The conventional double heterojunction FET 32 operates similar to the conventional heterojunction FET 10, wherein a two-dimensional electron gas formed in the channel layer 30 provides the channel of the device. The relaxation layer 28 is provided to offset the strain induced by the lattice mismatch of the barrier layer 14 and the channel layer 30 and thereby reduce the total strain of the conventional double heterojunction FET 32. To do so, the relaxation layer 28 is provided with an intrinsic lattice strain that is opposite the strain induced by lattice mismatching between the barrier layer 14 and the channel layer 30. Accordingly, while the barrier layer 14 and the channel layer 30 are strained layers, the relaxation layer 28 is not. In various embodiments, the relaxation layer 28 is aluminum gallium nitride. After providing the relaxation layer 28, the total strain of the conventional double heterojunction FET 32 is proportional to the compositional difference between the barrier layer 14 and the relaxation layer 28. The decreased strain provided by the relaxation layer 28 allows for an increase in the aluminum content of the barrier layer 14, which in turn theoretically allows for an increase in the sheet charge of the two-dimensional electron gas at the heterojunction of the barrier layer 14 and the channel layer 30.
In reality, however, the relaxation layer 28 also interacts with the channel layer 30 such that polarization occurs at the interface between these layers. Specifically, a two-dimensional hole gas is formed at the heterojunction between the relaxation layer 28 and the channel layer 30. The two-dimensional hole gas may decrease the sheet charge of the two-dimensional electron gas, which, as discussed above, results in decreased output power of the device. For example, the sheet charge of the two-dimensional electron gas at the heterojunction between the barrier layer 14 and the channel layer 30 may be 9×1012 cm−2, which does not significantly improve on that achieved by the single conventional heterojunction FET 10. Additionally, scattering from the two-dimensional hole gas will significantly reduce the mobility of two-dimensional electron gas, which will result in poor device performance.
FIG. 3 is a graph illustrating the conduction band (EC), Fermi level (EF), valence band (EV), two-dimensional electron gas charge density (ns) and two-dimensional hole gas charge density (ps) as a function of a distance from the surface of the cap layer 16 opposite the barrier layer 30 for the conventional double heterojunction FET 32. As illustrated, the charge density of the two-dimensional electron gas at the heterojunction between the barrier layer 14 and the channel layer 30 is lower than the charge density of the two-dimensional hole gas at the heterojunction between the relaxation layer 28 and the channel layer 30. The sheet charge of the two-dimensional electron gas and the two-dimensional hole gas are achieved by integrating over the area of the charge density in FIG. 3, such that the sheet charge of the two-dimensional electron gas is 9×1012 cm−2 and the sheet charge of the two-dimensional hole gas is 1×1013 cm−2. As discussed above, the relatively low sheet charge of the two-dimensional electron gas will reduce the on-state performance of the conventional double heterojunction FET 32.
In light of the above, there is a need for a heterojunction FET with increased sheet charge for the two-dimensional gas without sacrificing the reliability thereof.